This disclosure relates to an apparatus and method for evaluating a formation surrounding a borehole using an x-ray generator. More specifically, this disclosure relates to a system for using x-rays to determine the density of the formation. The measurements are taken using a downhole tool comprising an x-ray generator and a plurality of radiation detectors. The x-ray generator is capable of emitting radiation with high enough energy to pass into the formation and allow for substantive analysis of radiation reflected and received at the plurality of radiation detectors. In one embodiment, a reference radiation detector is used to control the acceleration voltage and beam current of the x-ray generator.
Well logging instruments utilizing gamma ray sources and gamma detectors for obtaining indications of the density and photoelectric effect (Pe) of the formation surrounding a borehole are known. A typical device comprises a long sonde body containing a gamma ray radioisotopic source and at least one gamma ray detector separated by a predetermined length. The sonde must be as short as possible to avoid distortion due to irregularities in the wall of the borehole that would cause a longer sonde to stand away from the actual formation surface. Distortion also is caused by the mudcake that often remains on the wall of the borehole through which any radiation must pass. These problems must be addressed by any system with the purpose of determining the density of the formation.
The radioisotopic sources used in the past include cesium (137Ce), barium (133Ba), and cobalt (57Co) among others. The basic measurement is the response seen at a radiation detector when radiation is passed from the radioisotopic source into the formation. Some radiation will be lost, but some will be scattered and reflect back toward the detectors, this reflected radiation is useful in determining properties of the formation.
While this radioisotope source type of system can provide an accurate result, there are drawbacks to the use of a chemical source such as 137Cs in measurements in the field. Any radioactive source carries high liability and strict operating requirements. These operational issues with chemical sources have led to a desire to utilize a safer radiation source. Although the chemical sources do introduce some difficulties, they also have some significant advantages. Specifically, the degradation of their output radiation over time is stable allowing them to provide a highly predictable radiation signal. An electrical photon (radiation) generator would alleviate some of these concerns, but most electrical photon generators (such as x-ray generators) are subject to issues such as voltage and beam current fluctuation. If these fluctuations can be controlled, this would provide a highly desirable radiation source.
Prior systems have attempted to use low energy x-rays to determine formation density. Photons with energy less than 250 keV are unlikely to be scattered back and received by the tools radiation detectors. If a tube operating below 250 kV is used, the electron current required will typically be too great to produce density measurements with reasonable efficiency. Additionally, at energies of 300 keV and greater, the interaction with the formation is dominated by Compton Scattering. This type of interaction is desirable in the calculations required to determine the bulk density of the formation from the measurement of attenuated radiation.
Accordingly, a need has been identified for a tool that may be used to determine formation density downhole. The photon generator used must be stable over time with its parameters closely controlled to ensure accurate measurements regardless of changing conditions. The photon generator must be capable of providing significant amounts of radiation consistently with energies at or above 250 keV.